(a) Technical Field
The present invention relates to a cooling system for a low-pressure casting mold. More particularly, the present invention relates to a cooling system for a low-pressure casting mold, which can reduce cycle time with an improved cooling rate and improve properties of a material used.
(b) Background Art
Generally, a low pressure casting process is intended to gradually cast molten metal at low pressure from a lower portion of a mold, and to allow the molten metal to be solidified. Such a low pressure casting process is used to produce engine blocks, cylinder heads, wheels, etc., since the molten metal produces few casting defects with fewer foreign substances, such as oxides, and thus it is possible to produce precision casting products.
The low pressure casting process is used in casting aluminum alloys, as well as copper alloys and cast iron. For an aluminum alloy, an appropriate mold temperature is 300 to 400° C. Since the bottom of the mold is adjacent to a heat source, its temperature is naturally about 50 to 100° C. higher, and thus it is possible to achieve directional solidification.
A low pressure casting process is determined by a correlation between a change in pressure of a pressure tank and a back pressure in the mold space. The higher the casting rate, the higher the back pressure; however, the back pressure is negligible compared with the pressure of a tank if the gas is sufficiently exhausted.
Accordingly, in the casting process, it is necessary only to control the pressure of the tank.
A pressurizing process in the low pressure casting process is divided broadly into three steps. The first step is a process in which molten metal rises directly under a sprue in a feeding pipe (connection pipe) upon pressurization.
In this case, the connection pipe is kept warm using a gas burner to reduce a drop in temperature of the molten metal. Moreover, since the molten metal should rapidly rise in a state where air is not mixed by shaking of the molten metal or oxides, it is necessary to use a casting machine with ventilation capability.
The second step is a process in which the molten metal is cast into the mold space through the sprue. The casting rate should be high to prevent occurrence of whirl and should be low to prevent gas inclusion.
The third step is a solidification process after the molten metal is completely cast into the mold and is related to a riser effect. According to this step, it is preferable that the pressurizing force is high; however, if it is too high, a gas discharge hole may become clogged or a coating material may be peeled off.
When a sand core is used, it is necessary to control the shift timing from the second step to the third step and the pressure rate.
Accordingly, after the molten metal cast into the mold is completely solidified, the molten metal in the feeding pipe that is not yet solidified is returned to a molten-metal holding furnace by eliminating the pressure exerted thereon, and the mold is opened to enable the molded product to be extracted.
FIG. 1 is a schematic diagram showing an exemplary conventional low-pressure casting apparatus for aluminum products, in which a mold is disposed at an upper portion and a casting means for casting molten metal is disposed at a lower portion.
The mold is divided into an upper mold 1 and a lower mold 2, in which the upper mold 1 is connected to a moving plate 3 moving up and down.
The casting means includes a tank 6 having a predetermined volume, in which a pressure gas supply inlet 4 is formed on one side and a molten metal filling inlet 5 is formed on the other side, a furnace 7 disposed on the bottom surface of the tank 6, and a casting passage 8, through which the molten metal in the furnace 7 is cast into a cavity of the mold, connected between the furnace 7 and the cavity of the mold.
Accordingly, at the same time when gas is supplied into the tank 6 through the pressure gas supply inlet 4, the pressure of the gas is exerted on the surface of the molten melt in the furnace 7 and, subsequently, the molten melt is cast into the cavity of the mold through the casting passage 8. After the molten melt cast into the mold is completely solidified, the pressure is removed to allow a molded product to be extracted.
FIGS. 2 and 9 are diagrams illustrating the position of a sprue 11 in a conventional low-pressure casting mold for a cylinder head 10, in which the sprue 11 is preferably located on a lower surface of the cylinder head 10, and thus the direction that the molten metal is cast is from the bottom to the top. Accordingly, an overhead gate 22 is formed at the bottom of the conventional sprue 11.
In this case, the molten melt is directionally solidified from the diagonally opposite side of the gate 22 to the gate 22, i.e., solidified from the upper surface to the lower surface of the cylinder head 10.
Moreover, after the molten metal is filled in the mold, the cylinder head 10 is solidified by air cooling through the upper and lower molds 1 and 2.
FIG. 3 is an exemplary perspective view showing a conventional connection pipe. The connection pipe 12 connects a casting furnace to a mold so as to cast molten metal in the casting furnace provided at the bottom to a cavity of the mold. A plurality of sprues 12a and 12b is preferably formed in the inside of the connection pipe 12 such that the molten metal is cast into the cavity of the mold through the sprues 12a and 12b. 
Preferably, the connection pipe 12 should be kept warm so that the molten metal is cast at a predetermined temperature. Suitably, conventionally, the periphery of the connection pipe 12 is heated by a gas burner.
However, it is difficult to adjust the temperature of the gas burner, and it is also difficult to cool the overheated mold, and the energy cost required to operating the gas burner is high.
FIGS. 4A and 4B are diagrams showing an exemplary structure of a conventional lower mold 13 and, as shown in the figure, the conventional lower mold 13 is of an overhead gate type in which the distance between a combustion chamber 13b and a sprue 13a is short.
However, there is insufficient space for installing a cooling system for the combustion chamber as shown in the above structure, and the sprue may be clogged in the event of overheating, and the combustion chamber is not cooled.
In particular, as shown in exemplary FIGS. 5A and 5B, a cooling groove 14 is provided at a portion where hot spots are formed on the lower surface of a lower mold 13 to cool the hot spots between the sprues 13a by air, and a cooling block 15 assembled with two pipes 16a and 16b in both directions of the cooling groove 14 is connected to the cooling groove 14. Two inlets and outlets are formed in the up and down direction of the cooling block 15 so that air supplied through a cooling pipe 16a provided on one side is introduced through the inlets of the cooling block 15 to cool the lower surface of the lower mold 13 and is then discharged through the outlets of the cooling block 15 to a cooling pipe 16b provided on the other side.
The above-described structure can eliminate shrinkage defect; however, the cooling effect is reduced.
Moreover, as shown in exemplary FIGS. 6 and 7, when gas is introduced and discharged through an inlet 18a and an outlet 18b, formed on the side surface of a conventional mold 17a, the gas is not cooled and naturally discharged. Reference numeral 17b denotes a mold cover.
FIG. 8 shows a cooling structure of a conventional upper mold, in which air cooling is performed to eliminate shrinkage defect of a spark plug 19; however, the shrinkage defect occurs intermittently, and an upper mold 20 is not efficiently cooled. Reference number 21 is an air cooling pipe.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.